The cardiac response to chronic stress involves the activation of a signal transduction network that induces myocyte hypertrophy and that in disease promotes myocardial apoptosis and interstitial fibrosis contributing to the development of heart failure. The phosphatase calcineurin (CaN) plays a central role in these processes. In cardiac myocytes there are two different isoforms of the CaN catalytic subunit, A? and A?, and yet only A? is required for myocyte hypertrophy. Because CaN isoforms have similar substrate specificity and share common binding partners, it is unclear why there would be functional differences between the isoenzymes. We recently performed a screen for CaNA?-specific binding partners. One protein that bound CaNA specifically is CIP4, a member of the F-BAR family of membrane-associated proteins. With the exception of our recent publication showing that CIP4 is required for the hypertrophy of cultured neonatal ventricular myocytes, F-BAR proteins have not been studied in the heart. In this application, we test the hypothesis that by serving as a CaNA? scaffold, CIP4 contributes to the regulation of cardiac myocyte hypertrophy. We propose that CIP4 binding co-localizes CaNA? with upstream receptors, ion channels and/or other signaling molecules that specifically confer CaNA? function in pathologic cardiac remodeling. Specific Aim 1: The Requirement for CIP4 in Cardiac Remodeling In Vivo. In light of our in vitro findings, we propose to characterize the cardiac phenotype of a new conditional, cardiac-specific CIP4 knock-out mouse. The mice will be studied both when unstressed and when subject to chronic transverse aortic constriction, ?-adrenergic, and angiotensin II receptor stimulation. We anticipate that the CIP4 knock-out mouse will have relatively normal cardiac function when unstressed and will be protected from the cardiac remodeling induced by pressure overload and neuroendocrine stimulation. Specific Aim 2: Mechanisms underlying CIP4-CaNA signaling in myocytes. Using novel FRET biosensors that will detect Ca2+ and CaN activity at CIP4 complexes, we propose to study the regulation of CIP4-bound CaNA? in cultured adult and neonatal myocytes. We will study the co-localization of CIP4 and CaNA? in the myocyte and test whether, as in other cell types, CIP4 localization may be regulated by growth factor and G-protein coupled receptor stimulation and by Rho family GTPases. In addition, we will determine whether gene expression regulated by the CaN effectors NFATc and MEF2 requires CIP4 expression in vivo. Specific Aim 3: Therapeutic Disruption of CaNA? Anchoring In Vivo. The central hypothesis of this application is that the unique role of CaNA? in pathological myocyte hypertrophy is due to CaNA? anchoring by specific scaffolds. We propose to express selectively in the cardiac myocyte anchoring disrupter peptides for CaNA? in order to test whether disruption of CaNA? anchoring can prevent pathological remodeling in response to long-term pressure overload and catecholamine infusion.